A fracture of the turbine shaft of a turbomachine, fortunately very infrequent in practice, can be due to poor fitting or poor protection of the shaft from oxidation.
At the time of fracture of this shaft, which connects the rotor of the turbine to the fan of the turbomachine, the moving blades of the turbine are still driven in rotation by the gases emerging from the combustion chamber of the turbomachine but are detached from the fan which was limiting their speed of rotation. The turbine then races and goes into “overspeed”, which subjects the moving blades to excessive stresses able to cause an explosion of the rotor with risks of perforating the outer casing of the turbine and of perforation of the fuselage of the aircraft fitted with this turbomachine. Limiting overspeed is therefore a major constraint to be complied with in turbomachines.
The known devices for limiting overspeed generally use the downstream displacement of the rotor of the turbine which results from the fracture of the turbine shaft and the pressure of the gases on the blades of the rotor.
Devices have already been proposed for the mechanical braking of the turbine rotor, comprising means carried by the rotor and intended to come to bear on corresponding means of the stator in such a way as to brake the rotor, following its downstream displacement after the fracture of the turbine shaft.
These devices have the disadvantage of being relatively slow, which acts against their efficiency.
It has also been proposed to fit guide blades of the stator in a detachable or pivoting manner so that the rotor, during its downstream displacement after the fracture of the turbine shaft, comes to bear on these blades and causes them to pivot over the path of the moving blades in order to destroy them and thus to slow down the rotation of the turbine. This known solution is however complex and costly. It is also necessary to provide means preventing the pivoting of these guide blades for causes other than the fracture of the turbine shaft.
Furthermore, the known devices generally have the disadvantage of increasing the overall mass of the turbine and of modifying the aerodynamic profile of its components.